In a typical nuclear reactor pressure vessel, a removable closure head is secured to the pressure vessel by a multitude of stud bolts, each having an associated nut and washer. One design of a typical four-loop plant has 54 such stud bolts. Stud bolts typically have threaded sections at both ends: the lower threaded section passes through an aperture in the vessel head and is received within a threaded bore in the reactor vessel flange; and the upper threaded section projects above the vessel head, upon which section the nut is torqued down against an associated washer and the vessel head to compress a seal between the head and the vessel. In this manner, the vessel head is securely held in sealing engagement with the reactor vessel.
For uniform nut loading on the studs, the studs are tensioned by a process well known in the art. The studs are tensioned and the nuts securely threaded thereon and torqued at a predetermined level, which procedure prevents inadvertent loosening of the nuts during reactor operation. A measuring rod is received within a vertical bore which extends the entire length of the stud, and is used to measure stud elongation to ensure proper tensioning. After the head has been so installed the vertical bore is sealed by means of a screw or bolt which is threaded into the top portion of the stud in order to prevent accumulation of water or other material therein.
Projecting through the top of the vessel head are a plurality of control rod drive mechanism housings. For a typical four loop plant, there are about 80 such ports. An example of one such design is shown in FIGS. 1A and 1B. Normally, there are on the order of 60 fuel assemblies out of the approximately 200 within the reactor vessel core which have control rods associated therewith. Depending upon such factors as power level of the reactor, enrichment and depletion of fuel in the core, control rod drive mechanisms are inserted through preselected housings. The remaining housings are utilized as instrumentation ports or spare penetrations. The instrumentation ports are for the introduction of instrumentation devices, such as thermocouples, into the reactor vessel.
The control rod drive mechanism housing is a stainless steel pressure housing attached to a head adapter or port projecting upward from the reactor vessel head. The adapter is welded to the reactor vessel head and constitutes, in effect, an integral part of the vessel. A typical pressurized water reactor is operated at an internal pressure of about 15 MPa (2250 psia); the design pressure of the reactor vessel and associated components is about 17 MPa (2500 psia). Control rods secured to the lower end of the control rod drive mechanism are periodically inserted into and withdrawn from a fuel assembly, depending on power demands in the reactor core. Reactor coolant water may leak from the mechanical flange joint on the housing because of such large internal pressures. When the drive mechanism is withdrawn, a thin film of liquid coolant around the drive mechanism may be withdrawn with it. This coolant may then drip onto reactor vessel components, specifically the exposed portion of the stud bolt projecting above the vessel head. Therefore, although the vessel and associated components are welded and sealed as best as they can be, it may be possible for some liquid coolant, typically water, to drip from the control rod drive mechanism housings.
The coolant within the reactor vessel is slightly acidic due to the presence of boric acid which is dissolved within the coolant. Boric acid is a neutron absorber used as a variable reactivity control over the long term operation of the plant, whereas the control rods provide rapid reactivity control for shutdown and other rapid reactivity changes. The number and placement of control rods is dependent upon numerous operating characteristics of a nuclear power plant.
Even though there are regulatory limits on the allowable amount of coolant which be emitted from the reactor vessel, components on the exterior and in close proximity to the reactor vessel need to be protected from any possible corrosive leakage impingement due to the presence of borated water. One purpose of the top closure screw, referred to previously, disposed within the bore of the stud is to prevent accumulation of this potentially corrosive borated water within the stud. Although the internal vertical bore of the stud bolt is sealed off from this corrosive spray leakage, the exposed external threads of the stud projecting above the vessel head, as well as the associated nut and washer, are not.
If the exposed threads are damaged, it may become difficult to quickly and easily remove the nuts therefrom. This is undesirable since removal and replacement of the vessel head is to be completed in as short a time period as possible. First, since the reactor vessel in a containment building of a nuclear power plant defines an irradiated environment, it is advantageous to provide for rapid maintenance procedures to reduce the time in which maintenance personnel are required to spend in and around such environment, thereby reducing individual man-rem exposure. Secondly, the quicker and easier such routine maintenance can be performed, the less down time experienced by the nuclear reactor. Since a nuclear reactor power plant operator cannot generate electricity when the plant is not operating, it must purchase replacement power elsewhere. Plus the more rapidly routine maintenance can be completed, that much more time can be devoted to actual power plant operation thereby improving the efficiency of the plant as well as increasing revenues generated by its operator.
There exists in the prior art conventional protective caps for bolts and nuts. An example of which is U.S. Pat. No. 3,548,704 issued to Kutryk on Dec. 22, 1970. Although such devices are satisfactory for use in a normal, everyday environment, a nuclear power plant presents additional concerns and considerations. Such a plant is a highly sophisticated and complex machine, damage to which must be prevented with great diligence. Also, any coolant that may be emitted from the reactor vessel needs to be safely dealt with and controlled, and not allowed to merely run off the protector to the environment. Thus a stud protector for use in association with a nuclear power plant must not only be a reliable device which cannot be accidentally removed, but must also be able to control the leak path o the coolant. It must be able to withstand the relatively harsh environment, especially with respect to the borated coolant. Furthermore, it is highly desirable to have available a stud thread and nut protector which can add significantly to other safety aspects associated with a nuclear power plant.
It is therefore an object of the present invention to provide a reactor vessel stud thread and nut protector which can be used in the highly specialized environment of a nuclear reactor pressure vessel.
It is another object of the present invention to provide such a device which will prevent damage to exposed threads and nuts resulting from potentially corrosive spray leakage of borated and irradiated coolant.
It is a further object of the present invention to provide a reactor vessel stud thread protector which can also collect any such coolant for return to the reactor's coolant system.
It is a still further object of the present invention to provide a device which will provide protection from accidental deformation to the stud, nut and washer due to operator maintenance around adjacent studs.